Runtime can't share database.
The file need to be hosted by FileMaker Pro or Server, and each PC need Pro for share it.
Thank you for your post!
As user19752 mentioned, Runtimes are not capable of sharing standalone.
I'm also going to move this thread from the FileMaker Community Feedback Space to the Discussions Space where others might find your post easier and offer further advice.
Also, if the users are accessing the file by opening it across the network, you will end up with corruption in the database eventually. There is a proper way to access a shared database with FileMaker. Search the knowledge base on filemaker.com.
This requires multiple licenses of FileMaker. Setting up a Computer to Share Files with FileMaker Pro | FileMaker